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Figure 6.28

Radial-line
Regular Triangular Pyramid. Fig. 6.29 represents the plan and elevation of a regular
triangular pyramid, and its development. If face ABC is placed on the plane, its true size will be
shown in the development. The true length of the base of triangle ABC is shown in the plan. As the
slanting edges, however, are not parallel to the vertical, their true length is not shown in elevation
but must be obtained by the method of rotation (for example), as indicated in Fig. 88. The triangle
may now be drawn in its full size at SAC in the development, and as the pyramid is regular, two
other equal triangles, SCB and SAB, may be drawn to represent the other sides. These, together
with the base ABC, constitute the complete development.

Figure 6.29 Plan, Elevation of Triangular Pyramid and Development of Triangular Pyramid.

Cone. If a cone be placed on its side on a plane surface, one element will rest on the surface. If
now the cone be rolled on the plane, the vertex remaining stationary until the same element is in
contact again, the space rolled over will represent the development of the convex surface of the

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